The present invention relates generally to communication systems, and more particularly to a system and method for providing optical amplification using input signals having launch powers that are a function of the noise figure of at least a portion of the system.
In designing a wavelength division multiplexed optical transmission link including multiple spans of fiber with optical amplifiers interposed between the spans, conventional design approaches assume that the noise figure for the system is spectrally flat and equal in magnitude to the worst case noise figure for the system. Designers calculate a desired signal to noise ratio (SNR) as a function of the number of spans in the system, and select a launch power for wavelength signals input to the system that ensures that all channels will achieve the desired SNR, even at wavelengths having the highest noise figure. Generally, designers apply the same launch power to all wavelength signals.
The present invention recognizes a need for a more efficient optical communications system and method of communicating signals.
In one embodiment, an optical amplifier comprises a gain medium operable to receive a plurality of signals each comprising a center wavelength and a noise figure associated with at least a portion of the amplifier and varying as a function of wavelength. At least two of the plurality of signals comprise a launch power that is a function of a magnitude of the noise figure measured at or near the center wavelength of that signal.
In another embodiment an optical amplifier comprises an input operable to receive a plurality of signals each comprising a center wavelength, wherein at least two of the plurality of signals comprise different launch powers. The amplifier further comprises a pump operable to generate a pump signal and a gain medium operable to receive the plurality of signals and the pump signal and to facilitate amplification of at least some of the plurality of signals. The amplifier also comprises an output operable to communicate amplified versions of the plurality of signals. A signal to noise ratio measured at the output of the amplifier varies by no more than 2.5 decibels over a bandwidth of at least 40 nanometers for at least a majority of signals output from the amplifier.
In still another embodiment, an optical communication system comprises an input terminal comprising a plurality of optical transmitters each operable to output one of a plurality of signals each comprising a center wavelength. The system further comprises a plurality of spans of optical medium coupled to the input terminal and operable to facilitate communication of the plurality of signals and a plurality of in-line amplifiers each coupled to at least one of the plurality of spans of optical medium. At least some of the plurality of signals comprise a launch power that is a function of a noise figure associated with at least a portion of the system.
In a method embodiment, a method of communicating optical signals comprises communicating a plurality of signals each having a center wavelength to an optical link comprising a plurality of spans of fiber. The method further comprises amplifying the plurality of signals to at least partially compensate for losses in one or more of the plurality of spans of fiber. Signals output from the optical link experience a noise figure varying as a function of wavelength. At least two of the signals input to the optical link comprise a launch power that is a function of the noise figure measured at or near the center wavelength of that signal.
In another method embodiment, a method of communicating signals comprises adjusting launch powers of a plurality of signals input to an optical link based at least in part on a noise figure associated with at least a portion of the optical link. The method further comprises adjusting a pump power of an amplifier in the optical link to give a desired gain spectrum in light of the adjusted launch powers. The steps of adjusting the launch power and adjusting the pump power are repeated until a signal to noise ratio at an output from the optical link varies by no more than a threshold amount for at least a majority of signals output from the optical link.
Depending on the specific features implemented, particular embodiments may exhibit some, none, or all of the following technical advantages. One embodiment provides a mechanism for reducing the total launched signal power in an optical link. Reducing the launched signal power reduces the intensity of light on connectors and other components, increasing the reliability of the system. Additionally, reduced launched signal power allows for use of lower powered pumps in amplifiers within the system. Reducing the pump power required generally results in decreased system costs.
As an additional benefit this technique facilitates freedom in design of gain profiles in multiple stage amplifiers. Because signal launch power is selected to at least partially address the noise figure issue, gain profiles of the amplifiers can be selected with less regard to maintaining a particular noise figure shape or magnitude.
Although this technique applies to and benefits many amplifier types, at least the following additional advantages can be realized when applying this technique to systems using Raman amplification. For example, when implemented in a Raman amplification system, this technique results in reduced non-linear penalties, such as four-wave mixing and Brillouin) effect, which tend to be less prevalent at lower signal powers.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, description and claims.